Solid state image display devices utilizing emissive pixels are well known and widely used. Much work has been done to improve the brightness, uniformity, contrast, etc. of the displays so as to make them as pleasing as possible. For example, European Patent Application EP 0 905 673 A1, by Kane et al., published Mar. 31, 1999, entitled "Active Matrix Display System and a Method for Driving the Same" and the article entitled "A Poly-Silicon Active Matrix Organic Light Emitting Diode Display with Integrated Drivers" by Dawson et al., published in the society for Information Display Digest, 1998, pp. 11-14, describe such efforts. Generally speaking, these devices require power to maintain their information state (they are volatile) and because of charge leakage, can only maintain and display an image for a limited amount of time after which it begins to fade (they are not persistent). The image is then refreshed, that is the image is rewritten into the display device. Refresh circuitry can be complex, require high data rates, and impose a significant cost and size burden on a system. In particular, refreshing a display requires a significant use of system power. The frequency with which the display must be rewritten depends on the persistence of the display (how long it can maintain an acceptable image) and the rate at which the image content changes. If the image content changes more frequently than the rate at which the image fades, there will never be a problem. This is generally the case in video-rate systems. However, in cases where the content changes slowly or where only portions of an image change, frequent display refreshes may be unnecessary. Indeed, a persistent imaging system designed for still images alone may not require periodic refresh capability.
Solid-state displays can be characterized as emissive or non-emissive. An emissive display directly generates light at each pixel and requires power to operate and display information. Liquid crystal displays (LCDs), in contrast, are non-emissive and maintain their state without drawing significant current. (LCDs are non-volatile although power is needed to make their state visible either through back-lighting or ambient light, or to change their state. The switched state is maintained through an applied electrostatic field.) The liquid crystals themselves do not emit light but rather change the polarization of light passing through them. LCDs are thus non-emissive and generally utilize a back-light to make their display visible. A non-volatile display is, by definition, persistent.
Solid-state image displays are typically organized by address and data controls representing the value of each pixel in the display. The address is converted into a select line (or combination of select lines) controlling an individual pixel and a data line representing the analog value of the pixel. Each pixel is then managed by the Data and Select control lines and incorporates means to store a charge representing the value of the pixel at the pixel site, and a mechanism to emit light from the stored charge. The control mechanisms are generally implemented using transistors and the storage mechanisms through capacitors. U.S. Pat. No. 5,552,678 issued Sep. 3, 1996 to Tang et al., entitled "AC Drive Scheme for Organic LED" describes a specific drive scheme for an implementation using organic LEDs.
FIG. 1 represents a generic diagram implementing a display pixel in an LED display. In this figure, the pixel 10 has a control mechanism 12 that stores charge in a capacitor 14 which then drives a display mechanism. The transistor Tc 12 is responsive to the control lines (Data 16 and Select 18) and, when active, deposits a charge into Cref 14. Cref then controls the driver, Td 20, for an LED display component 22. Td 20 is optimized to effectively drive the LED 22; Tc 12 to charge the storage capacitor 14 and respond to the control lines 16 & 18. To perform these tasks, both transistors 12 & 20 tend to be large; Tc 12 to provide fast switching time and Td 20 to provide the maximum current (and brightness) through the LED 22.
The persistence of the display is directly related to the length of time that the storage capacitor can maintain its charge. There are three basic mechanisms through which this charge can dissipate. The first leakage path is directly across the capacitor indicated by arrow 24 and will be affected by the materials and structures used to implement the device. Second, charge is used to drive the display mechanism which provides a second leakage path indicated by arrow 26. Third, charge can leak back through the control mechanism indicated by arrow 28. These leakage paths are illustrated with the curved arrows in FIG. 1. Leakage through the capacitor itself is exacerbated by material impurities; leakage back through Tc is attributed to source-to-drain and source-to-gate leakage; and through Td by gate-to-source leakage. The leakage through the transistors is greater for larger transistors.
Because of the inherent loss of charge at each pixel site in a display device, the devices must be periodically refreshed, i.e. the image data must be rewritten to the display. FIG. 2 illustrates a generic system. As shown in FIG. 2, an imaging system 40 includes a display device 42, a refresh circuit 44 and a control circuit 46. The refresh circuit 44 receives a periodic signal 48 instructing it to refresh the image display. The need for periodic refresh in an image display system for displaying still images imposes system costs by enforcing potentially unnecessary refresh requirements. These system costs can include design effort, manufacturing costs, complexity, performance, reduced system reliability, and power. There is a need therefore for an improved image display with reduced refresh needs that is less costly to manufacture, has a simpler design and exhibits improved performance over the prior art devices.